The effect of host variables such as size and density, on the transmission of cercariae of Diplostomum spathaceum (Rudolphi, 1819) into a second intermediate fish host, rainbow trout, Oncorhynchus mykiss (Walbaum), was studied quantitatively in laboratory experiments under varying exposure conditions. Differences in the number of established metacercariae were noted in differently sized fish exposed singly to the same number of cercariae and in water volumes related to the body surface area of the host. When exposed (1) singly in an equal water volume or, (2) simultaneously in the same tank, no differences in recovery of metacercariae per fish were found between “small” and “large" hosts. The latter observation is valid for three water volumes tested using the same number of cercariae per host. No significant difference in metacercarial infection was revealed in similar sized fish exposed at different host densities. It appears that transmission is more influenced by cercarial density (number of cercariae per water volume) and fish size than by fish density. These experimental findings support the view that cercarial infection offish occurs by chance, presumably mainly in the gill region. Accordingly, in the field, individual fish size and cercarial density rather than fish population density, prevail in parasite transmission from snail to fish. This is of interest in coastal areas heated by cooling water, where fish growth and fish population density are enhanced.
Cercariae of Cotylurus flabelliformis (Faust, 1917) were individually tested in 6 experiments for evidence of chemoattraction to snail hosts, host-speeificity, and rate of dispersal. Five species of snails were tested: Lymnaea stagnalis appressa Say, 1821, Stagnatola eludes (Say, 1821), Physella gyrina (Say, 1821), Planorbella trivnlvis (Say, 1817), and Oxyloma retusum (Lea, 1834). The data substantiate that cercariae of C. flabelliformis have a chemo-positive attraction to undetermined diffusable substances from snails, show host-specificity, and do not attack the snail that produced them. Cercarial response time was significantly influenced by cercarial age (post-emergence), and duration of presence of snails. Newly emerged cercariae were most consistently chemo-positive to L. stagnalis and quickly located all snail species except the unnatural host O. retusum. Tests of 121 cercariae proved statistically significant in showing preferences for snail hosts. Cercarial dispersal experiments showed that C. flabelliformis cercariae can disperse rapidly but few located and penetrated lab-reared S. eludes at distances up to 1.2 m. The results are consistent with other studies which indicate that chemical gradients are used by cercariae to locate slow moving hosts and that cercariae must come into close proximity of snails before attraction occurs. The significance of these observations to the distribution of C. flabelliformis in the molluscan population is discussed.
Examination of 4055 molluscs of 10 species from cenotes (= sinkholes) and other freshwater bodies in the Yucatan Peninsula, Mexico revealed the presence of-larval stages of 13 trematodes. The following species were found: Echinochasmus leopoldinae Scholz, Ditrich et Vargas-Vázquez, 1996, E. macrocaudatus Ditrich, Scholz et Vargas-Vázquez, 1996 (Echi-nostomatidae), Saccocoelioides sp. (? sogandaresi Lumsden, 1963) (Haploporidae), Crassicutis cichlasomae Manter, 1936, pleurolophocercous ophthalmocercaria sp. (Homalometridae), Ascocotyle (Ascocotyle) sp., Ascocotyle (Phagicola) nana Ransom, 1920 (Heterophyidae), Oligogonotylus manieri Watson, 1976 (Cryptogonimidae), Genarchella astyanactis (Watson, 1976) (Derogenidae), xiphidiocercariae sp. 1, 2 and 3 (Lecithodendriidae?), and furcocercaria gen. sp. (Fellodistomidae). The life-cycle of the derogenid Genarchella astyanactis was studied for the first time. It was found that it differs from that of G. genarchella: the first intermediate host, Pyrgophorus coronatus (Pfeiffer, 1839), released cystophorous furcocercariae of G. astyanactis that developed, after ingestion by the second intermediate host, copepods (experimentally Mesocyclops chaci Fiers, Reid, Ilife et Suárez-Morales, 1996), into metacercariae resembling by their morphology juvenile trematodes found in the stomach of Aslya-nax fasciatus. No progenetic cercariae (metacercariae) found in G. genarchella were observed in the life-cycle of G. astyanactis. Rediae and cystophorous furcocercariae were recovered from naturally infected snails and snails experimentally kept in contact with eggs from the uterus of G. astyanactis adults.
Schistosoma monsoni (Puerto Rican strain) cercariae were exposed to 10, 20, 50, 100, 200, 300, 400 and 500 krad gamma radiation from “Co gamma source. From 10-100 krad irradiation no differences could be found in the behaviour of the cercariae when compared to the controls. From 200-500 krad there was an increase in mortality as well as in the number of cercariae that shed their tails. The Con A binding studies also performed on cercariae radiated with up to 100 krad indicate a direct relationship between the number of cercariae that bind Con A and radiation dose. All live cercarial heads collected after radiation also bound Con A. It thus seems possible that irradiation may act as a stimulus for cercariae to transform to schistosomulae.
The human infection known under the names cercarial dermatitis or swimmers' itch is generally associated with swimming in lakes all over the world, however, a number of outbreaks of cercarial dermatitis developing in salt or brackish waters are also reported. The disease presents as allergic reaction which is able to trap and eliminate the parasites in the skin. However, the infection can be linked to more than skin symptoms under certain circumstances. Recent studies on bird schistosomes have shown that during primary infections of noncompatible hosts (mice) the parasites may migrate through visceral and nervous tissues of mammals. Up to date, cercarial dermatitis has been mostly associated with the cercariae of bird schistosomes of the genus Trichobilharzia. Recent findings of new genera and species indicate, however, broader spectrum of causative agents of the disease with different life cycles, host specificity and pathogenicity.
Of 14,431 Clypeomorus bifasciata Sowerby, 1855 collected from Kuwait Bay between November 1992 and October 1993, 718 (5.0%) were shedding a total of 12 morphologically distinguishable ‘types’ of cercariae: 1 cystocercous, 2 echinos-tome, 1 furcocystocercous, 3 magnacercous, 2 megalurous, 1 microcercous and 2 ubiquità. Ubiquità I and furcocystocercous were the most prevalent species accounting for 66% and 25%, respectively, of total cercaria fauna. Double infections were recorded in only 6 (0.8%) of the infected snails. The overall prevalence of C. bifasciata shedding cercariae was highest (6-10%) in spring and summer, and species diversity was highest (11 species) in fall and spring. Seasonal variations in prevalence and diversity of cercariae were related to environmental factors and hosts behavior.
The toxicity of cadmium and zinc at concentrations ranging from 0.1 to 100 µg/l was investigated against the activity of Diplostomum spathaceum (Rudolphi, 1819) cercariae. Over a 24 h exposure period a significant reduction in cercarial activity occurred in solutions of cadmium, zinc, and a mixture of cadmium and zinc at all concentrations. Reduced cercarial activity also occurred in all toxicant solutions compared with controls after only 6 h exposure indicating that cercariae were vulnerable during the period of maximum cercarial infectivity (0-5 h). The mechanisms of metal toxicity and their importance to parasite transmission are discussed.